Nine children, 10-15 years old with chronic asthma, were treated for weekly periods with gradually increasing doses of oral terbutaline sulphate: 45, 79, 118 and 166 micrograms/kg (mean values) 3 times daily. There was a linear relationship between dose and steady-state plasma concentration of terbutaline within patients, but the plasma levels varied 3-fold between patients taking similar doses. Symptom score, peak expiratory flow rate (PEFR) and volume of air expelled in the first second of forced expiration (FEV1) improved with increasing doses, and the need for inhalation therapy decreased. An increase in pulse rate and tremor was measurable at all dose levels, but reported side-effects were few and mild. Linear regression analysis showed a statistically significant relationship between the plasma concentration of terbutaline and the effect on FEV1 (p less than 0.01) and PEFR (p less than 0.05) within patients.